Method of molding large thin parts from reinforced plastic material

ABSTRACT

Reinforced plastic pellets comprise thermoplastic material and reinforcement particles that are less than 15% of a total volume of the pellets, and at least 40% have a thickness of less than 50 nanometers. A manifold ( 56 ) has at least two spaced valve gates ( 64 ) that are independently opened and closed as directed by a controller ( 68 ) to selectively communicate the manifold to a cavity. A primary injection pressure is applied to the plasticized pellet material in the manifold ( 56 ) to fill the cavity through sequential opening and closing of the gates ( 64 ). A lower secondary injection pressure is applied to the material in the manifold to continue filling the cavity. The gates are closed to seal the manifold from the cavity when the cavity is filled. The material is held within the manifold in compression by the valves while the cavity is open to prevent expansion of the material.

This application is based on and claims the priority benefit of U.S.provisional patent application Serial No. 60/096,158, which was filed onAug. 11, 1998.

FIELD OF THE INVENTION

The present invention relates to injection molding methods andapparatus, and, more particularly, a sequential fill valve gatedinjection molding system for molding reinforced thermoplasticsparticularly suited for producing large, thin molded components.

BACKGROUND OF THE INVENTION

Recently, there has been an increase in the demand and applications forlarge molded plastic parts. As a result, some of these parts have becomequite complex. One example of this can be seen in bumper fascia forautomobiles. Design engineers are now integrating many features into thefascia such as grilles and light openings to reduce tooling andmanufacturing costs. Also, to save material, fascia are designed withthinner walls. Due to the complex cavity geometries and increased flowlength versus wall thickness ratios, it is often difficult to predictthe actual flow pattern that will take place during mold filling.Although design software may be used to help determine the most optimumprocessing conditions, gate locations, and hot runner diameters for abalanced fill, quite often the expected fill pattern is not realized inpractice as a result of variables such as steel dimension variations,mold temperature variations, and venting inadequacies, for example.Process engineers are therefore faced with a nonuniform fill which undercertain conditions may result in decreased dimensional stability of thefascia, as well as deficiencies in paint adhesion characteristics and/orother surface appearance concerns.

Improved processing techniques that provide more control over thefilling of large complex cavity geometries are required to meet theincreased demands presented by more modem molding standards. To improvepart quality, melt front advancement must be further controlled duringthe actual filling phase to achieve a more uniform filling and packingdistribution. In addition, there is a continuing interest in pursuingfurther time and cost efficiencies associated with part manufacture.

U.S. Pat. No. 5,762,855 discloses an injection molding system for largemolded components that may be used to enhance the quality of the finalmolded part in a timely and cost-efficient manner. Specifically, thatpatent discloses a method for molding large components in a mold havingat least one mold cavity. Plasticized material is introduced into acavity mold through a manifold. The manifold has at least two spacedvalve gates that are independently opened and closed as directed by acontroller to selectively communicate plasticized material from themanifold to the mold cavity at separate locations in the mold cavity.The controller directs the valve gates to sequentially open and closeduring the filling phase so as to achieve the desired melt frontadvancement within the mold cavity. Once the mold cavity has beenfilled, the valve gates are closed to effectively seal the manifold fromthe mold cavity. The closed valve gates thereby assist in allowing theplasticized material within the manifold to be held in compression whilethe mold cavity is open for removal of the molded component from themold cavity, so as to prevent appreciable expansion of the material thathas been found to result in imperfections, such as splay, in moldedproducts.

While the invention disclosed in the '855 patent is particularly usefulfor producing large, thin walled plastic parts, its usefulness islimited by the structural characteristics of the plastic materialconventionally used. That is, while the invention disclosed isparticularly suited for parts with large planar dimensions and thinwalls, the usefulness of the disclosed invention is limited by the factthat the parts produced can be only so large or so thin before the partslose their structural integrity and impact resistance.

Heretofore, in order to reinforce various thin plastic parts such asfascia, such parts would conventionally be reinforced by mineral fillersor glass fibers. However, such reinforcement has a deteriorating effecton impact resistance of the part. Moreover, the conventionalreinforcement materials are inadequate to enable the full benefits thatmight otherwise be achieved by the methodology disclosed by the '855patent.

SUMMARY OF THE INVENTION

The disadvantages of the prior art may be overcome by providing a methodfor molding large, thin components in a mold having at least one moldcavity. Reinforced plastic pellets are provided, which pellets compriseat least one thermoplastic material and reinforcement particlesdispersed within the at least one thermoplastic material, thereinforcement particles comprise less than 15% of a total volume of thepellets, and at least 40% of the reinforcement particles have athickness of less than about 50 nanometers. The reinforced plasticpellets are melted to produce plasticized material therefrom. Theplasticized material is communicated through a manifold to a cavitymold. The manifold has at least two spaced valve gates that areindependently opened and closed as directed by a controller toselectively communicate the plasticized material from the manifold tothe mold cavity at separate locations in the mold. A primary injectionpressure is applied to the plasticized material in the manifold to fillthe mold cavity through sequential opening and closing of the valvegates as directed by the controller. A secondary injection pressure isapplied to the plasticized material in the manifold to continue to fillthe mold cavity. The secondary injection pressure is less than theprimary injection pressure. The valve gates are closed to seal themanifold from the mold cavity when the mold cavity is filled. Theplasticized material is held within the manifold in compression whilethe mold cavity is open for removal of the molded component from themold cavity. The compression is maintained with the assistance of theclosed valve gates to prevent appreciable expansion of the material.

Other objects and advantages of the present invention will becomeapparent from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWING

A preferred embodiment of the present invention is described herein withreference to the drawing wherein:

FIG. 1 is a schematic representation of a valve-gated injection moldingsystem in accordance with one embodiment of the present invention;

FIG. 2 is a schematic representation of a prior art injection moldingmachine communicating with a main bore from which multiplethermally-gated drops depend to introduce plasticized material to a moldcavity;

FIG. 3 is a cross-sectional side view of an example mold where a coreportion and a cavity portion mate to form a mold cavity;

FIG. 4(a) is a cross-sectional side view of a preferred valve gatednozzle of the type used in accordance with one embodiment of the presentinvention, with the valve pin in the open position;

FIG. 4(b) is a cross-sectional side view of a preferred valve gatednozzle of the type used in accordance with one embodiment of the presentinvention, with the valve pin in the closed position;

FIG. 5 is a timing diagram to illustrate the molding cycle time of anexample application of a prior art injection molding system;

FIG. 6 is a timing diagram to illustrate the reduced molding cycle timerealized in an example application of the present invention;

FIGS. 7(a)-(e) illustrate five temporally-spaced schematic illustrationsof a mold cavity to show the melt front advancement during a 10-secondfill time in an example automobile bumper mold application of thepresent invention through sequential operation of six valve gates; and

FIG. 8 is a timing diagram to illustrate the relationship between thepressure applied to the melt and the operation of the valve gates in anexample embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 and 2 each illustrate an injection molding apparatus wherebynanoparticle reinforced plastic pellets 10 are fed from a hopper 12 intoa cylindrical channel 14, where the pellets 10 are transported along thelength of the channel 14 through the use of a reciprocating screw 16.Axial rotation of the screw 16 is achieved through a hydraulic motor 18.As the pellets 10 traverse the channel 14, they become heated by heaterbands 20 and, as a result, the pellets 10 melt and coalesce to form amelt pool 22. The melt pool 22 that resides upstream from the screw 16constitutes the shot of plasticized material in queue to be nextinjected through the mold manifold and into the mold cavity.

In accordance with the present invention, the pellets 10 comprise atleast one thermoplastic material and reinforcement particles dispersedwithin the at least one thermoplastic material. The reinforcementparticles comprise less than 15% of a total volume of the pellets 10,and at least 40% of the reinforcement particles have a thickness of lessthan about 50 nanometers.

In a more preferred embodiment, at least 50% of the reinforcementparticles have a thickness of less than about 20 nanometers, at least90% of the reinforcement particles have a thickness of less than about10 nanometers, and at least 99% of the reinforcement particles have athickness of less than about 30 nanometers.

The reinforcement filler particles, also referred to as “nanoparticles”due to the magnitude of their dimensions, each comprise one or moregenerally flat platelets. Each platelet has a thickness of between0.7-1.2 nanometers. Generally, the average platelet thickness isapproximately 1 nanometer thick. The aspect ratio (which is the largestdimension divided by the thickness) for each particle is about 50 toabout 300.

The platelet particles or nanoparticles are derivable from largerlayered mineral particles. Any layered mineral capable of beingintercalated may be employed in the present invention. Layered silicateminerals are preferred. The layered silicate minerals that may beemployed include natural and artificial minerals. Non-limiting examplesof more preferred minerals include montinorillonite, vermiculite,hectorite, saponite, hydrotalcites, kanemite, sodium octosilicate,magadiite, and kenyaite. Mixed Mg and Al hydroxides may also be used.Among the most preferred minerals is montmorillonite.

To exfoliate the larger mineral particles into their constituent layers,different methods may be employed. For example, swellable layeredminerals, such as montmorillonite and saponite are known to intercalatewater to expand the inter layer distance of the layered mineral, therebyfacilitating exfoliation and dispersion of the layers uniformly inwater. Dispersion of layers in water is aided by mixing with high shear.The mineral particles may also be exfoliated by a shearing process inwhich the mineral particles are impregnated with water, then frozen, andthen dried. The freeze dried particles are then mixed into moltenpolymeric material and subjected to a high sheer mixing operation so asto peel individual platelets from multi-platelet particles and therebyreduce the particle sizes to the desired range.

The pellets 10 utilized in accordance with the present invention areprepared by combining the platelet mineral with the desired polymer inthe desired ratios. The components can be blended by general techniquesknown to those skilled in the art. For example, the components can beblended and then melted in mixers or extruders. Preferably, the pellets10 are cut from an extruded rod of material.

Additional specific preferred methods, for the purposes of the presentinvention, for forming a polymer composite having dispersed thereinexfoliated layered particles are disclosed in U.S. Pat. Nos. 5,717,000,5,747,560, 5,698,624, and WO 93/11190, each of which is herebyincorporated by reference. For additional background, the following arealso incorporated by reference: U.S. Pat. Nos. 4,739,007 and 5,652,284.

Preferably, the thermoplastic used for the purposes of the presentinvention is a polyolefin or a blend of polyolefins. The preferredpolyolefin is at least one member selected from the group consisting ofpolypropylene, ethylene-propylene copolymers, thermoplastic olefins(TPOs), and thermoplastic polyolefin elastomers (TPEs).

The exfoliation of layered mineral particles into constituent layersneed not be complete in order to achieve the objects of the presentinvention. The present invention contemplates that at least 40% of theparticles should be less than about 50 nanometers in thickness and,thus, at least 40% of the particles should be less than about 50platelets stacked upon one another in the thickness direction. At thisextent of exfoliation, with a loading of less than 15% by volume, thebenefits of the nanoparticles begin to accrue with meaningful effect formany large thin part applications. For example, such loading ofnanoparticles will provide a desired increase in the modulus ofelasticity by about 50-70% over conventional fillers.

More preferably, at least 50% of the particles should have a thicknessof less than 10 nanometers. At this level, an additional increase ofabout 50-70% in the modulus of elasticity is achieved in comparison withthe 40% of less than 50 nanometer thick exfoliation discussed above.This provides a level of reinforcement and impact resistance which wouldbe highly suitable for most motor vehicle fascia applications.

Even more preferably, at least 70% of the particles should have athickness of less than 5 nanometers, which would achieve an additional50-70% increase in the modulus of elasticity in comparison with the 50%of less than 10 nanometer thickness exfoliation discussed above. Thisprovides ideal reinforcement and impact resistance for large thin partsthat must withstand greater degrees of impart.

Even more preferably, at least 50% of the reinforcement particles have athickness of less than about 20 nanometers, with at least 90% of thereinforcement particles having a thickness of less than about 10nanometers, and at least 99% of the reinforcement particles having athickness of less than about 30 nanometers.

It is most preferable to have as many particles as possible to be assmall as possible, ideally including only a single platelet.

As noted above, the preferred aspect ratio (which is the largestdimension divided by the thickness) for each particle is about 50 toabout 300. At least 80% of the particles should be within this range. Iftoo many particles have an aspect ratio above 300, the material becomestoo viscous for forming parts in an effective and efficient manner. Iftoo many particles have an aspect ratio of smaller than 50, the particlereinforcements will not provide the desired reinforcementcharacteristics. More preferably, the aspect ratio for each particle isbetween 100-200. Most preferably, at least 90% of the particles have anaspect ratio within the 100-200 range.

Generally, in accordance with the present invention, the pellets 10 andhence the parts to be manufactured should contain less than 15% byvolume of the reinforcement particles of the type contemplated herein.The balance of the part is to comprise an appropriate thermoplastic(preferably polyolefin) material and suitable additives. If greater than15% by volume of reinforcement filler is used, the viscosity of thecomposition becomes too high and thus difficult to mold.

Returning to the figures, the preferred sequential fill valve gatedinjection molding system is shown in FIGS. 1 and 4-8. The press is usedin the preferred embodiment to produce automobile facias, such as bumpercomponents for example. It will be understood, however, that other typesof large parts, such as those that typically weigh 4 or more pounds forexample, may similarly be manufactured through the use of the presentinvention.

As shown for example in FIG. 3, a typical mold 24 consists of a cavityportion 26 and a core portion 28. The cavity portion 26 and core portion28 mate with one another to form a mold cavity 30, and are held withsubstantial mold press forces to form an injection molded part when themold cavity 30 is filled. The movable section of the mold, whether thecavity portion 26 or core portion 28 for example, can be opened andclosed upon the stationary section to allow molded parts to be withdrawnfrom the mold 24.

FIG. 5 is a diagram to illustrate a typical 100-110 second cycle usedbefore the present invention to form an automobile facia component. Withreference now to FIGS. 2 and 5, the cycle begins with the mold clampbuilding pressure to a full pressure that is maintained during themolding of the part. Pressure on the melt pool 22 exerted by the screw16 creates an injection pressure that is used to fill the mold cavity 30with the melt pool 22 in queue in the manifold 32. The first stageinjection pressure exerted on the melt pool 22 by the screw 16 causesthe melt pool 22 to advance through the main bore 34 of the manifold 32.

Six heated drops 36-41 depend from the main bore 34 at spaced intervalsto simultaneously introduce the melt pool 22 into the molding cavity 30at six separate locations. Although the mold cavity 30 is filled throughsimultaneous advancement of the melt 22 through the six drops 36-41,balancing of the fill may be sought by varying the diameters of theinterior central channels of the respective drops 36-41.

Once the part has been substantially filled (e.g. 95% filled) duringthis first stage pressure, the injection pressure is lowered to a holdpressure whereby the mold 30 continues to fill simultaneously throughall six drops 36-41 at a reduced injection pressure. Full pressure ismaintained on the clamp to keep flash to a minimum.

Once the mold has been completely filled, additional material isplasticized upstream of the thermal gates 42 during a screw recoverystage to form the next shot of plasticized material in queue for thenext part cycle. Once screw recovery is complete, each of the thermalgates 42 on the depending drops 36-41 draws heat away from adjacent moldsteel so as to harden plasticized material at the tips of all six drops36-41. This hardening of plasticized material at the tips of the drops36-41 in turn seals the manifold 32 such that melt 22 is retained withinthe manifold 32 in anticipation of the next part cycle.

The molding cycle then reaches a decompression stage whereby theinjection pressure imposed on the melt 22 is relieved substantially oreven altogether through retraction of the screw 16. Decompression of themelt 22 in this way helps ensure that the thermal seals formed at thethermal gate locations 42 on the manifold 32 remain effective when theclamp is opened and the molded part is removed. It has been found,however, that decompression of the melt 22 at this stage allows gassesor other volatiles to expand in the melt 22 upstream of the thermalgates 42, which in turn often results in imperfections insubsequently-molded parts. Such imperfections may appear on the moldedproduct as surface splay or silver streaking, for example.

Once the manifold 32 has been thermally sealed at the various gatelocations 42 and the melt 22 has been decompressed, the clamp is openedand the molded part is removed. The clamp then closes in anticipation ofthe next part cycle.

The total time for the molding cycle described above is approximately100-110 seconds or more for an example molded automobile bumper part.

FIG. 6 is a second diagram to illustrate a reduced cycle time achievedthrough the preferred embodiment of the present invention in one exampleapplication. With reference to FIGS. 1, 6, 7(a)-(e), and 8, the moldcavity 30 described herein is a single-cavity automobile facia mold thatis used to produce an automobile facia, such as a bumper component forexample, formed from a PC/polyester material such as molded-in colorPC/Polyester blend, TPO, TPE, or TPU.

With reference now to FIG. 1, plastic pellets 10 are fed from a hopper12 into a cylindrical channel 14, where the pellets 10 are transportedalong the length of the channel 14 through the use of a reciprocatingscrew 16. The pellets 10 melt as they traverse the heated channel 14 andcoalesce to form a melt pool 22. The melt pool 22 that resides upstreamfrom the screw 16 constitutes the shot of plasticized material in queueto be next injected through the mold manifold 50 and into the moldcavity 30. Displacement of the reciprocating screw 16 is detected by apositional sensor 52, and the output 53 of the sensor 52 is supplied toa control system 54 for use as later described.

The mold clamp pressure builds to and maintains a full pressure. Themold cavity 30 fills in a sequential manner, as described below, withthe melt pool 22 in queue. The primary or first stage injection pressureexerted on the melt pool 22 by the screw 16 creates an injectionpressure that causes the melt pool 22 to advance through the main bore56 of the manifold 50. The primary injection pressure is preferably onthe order of 10,000 to 20,000 PSI (or 68.9 to 137.8 Mpa), depending uponthe viscosity of the selected material.

The six spaced drops 58-63 that depend from the main bore 56 areoutfitted with valve gates 64 that may be independently open and closedthrough operation of a control system 54, such that the introduction ofthe melt 22 into the mold cavity 30 through a particular drop may becontrolled independent of the other drops. Specifically, the mold ispreferably outfitted with a KONA Valve Gate Hot Runner System orequivalent Six manifold drops 58-63 provide for the introduction of themelt 22 into the single mold cavity 30 at six different locations. AKona SR20VG valve gate 64 or equivalent is located at each of the sixmanifold drops 58-63.

Each valve gate 64 is actuated by a hydraulic control unit 66. Acontroller 68, such as the machine controller for the mold press forexample, is programmed to provide through lines 70 the desiredsequencing and other control over the pin actuation at the individualvalve gate locations 64. The preferred controller 68 controls thevarious valve gates as a function of both cycle time and position of thescrew 16. The output of a positional sensor 52 on the screw 16 may beused by the controller 68 as a reference for determining theinstantaneous aft and fore position of the screw 16. The controller 68thereby may direct the valve gates 64 to operate in such a way so as toexhibit greater control over the molding process. In this way thecontroller 68 may, for example, systematically control the flow frontsof the melt 22 within the mold cavity 30, and may manipulate the valvegates 64 to apply a final packing pressure at the appropriate stage ofthe mold cycle to compensate for shrinkage of the plasticized materialaway from the mold wall as the material cools.

As is shown for example in FIGS. 4(a) and 4(b), each of the six valvegates 64 feature an adjustable valve pin 74 that may be independentlycontrolled by an appropriately-programmed control system 54. The valvepin 74 extends centrally along the length of the manifold drop, and canbe reciprocated in an axial direction. When the valve pin 74 isretracted within the central channel 76 of the manifold drop, as isshown for example in FIG. 4(a), the melt 22 may pass from the main bore56 down the central channel 76 of the drop around the valve pin 74, andout an aperture 78 at the end of the drop and into the mold cavity 30.When the valve pin 74 is moved by the control system 54 into position toplug and seal the drop aperture 78, as is shown for example in FIG.4(b), the melt 22 ceases to flow into the mold cavity 30.

This positive mechanical gate shut off capability provided by the valvepin 74 not only helps reduce or eliminate vestige on part surfaces, butalso allows the valve gates 64 of the various drops 58-63 to besequenced during the injection stage as provided by the presentinvention. The example mold cavity 30 illustrated in the figures fillssequentially through the six valve gated nozzles on the six manifolddrops 58-63, as is shown in FIG. 8. The drops 58-63 are spaced so as todistribute plasticized material across the mold cavity 30 to completelyfill the cavity 30 in an efficient manner. The control system 54operates the valve gates 64 in a predetermined sequential manner toobtain an efficient and balanced fill of the mold cavity 30.

The valve gating sequence used for the automobile fascia describedherein is shown for example in FIGS. 7(a)-(e) and 8. Specifically, twogated nozzles 64 located in the outer wing regions 84 and 86 of the moldcavity 30 (drops 58 and 63) are first to open at injection time =0seconds. The central four gated drops 59-62 remain closed and a primaryor first stage injection pressure delivers plasticized material into thewing portions of the mold cavity 30 through the outer two gated drops 58and 63. At approximately 3.5 seconds into the injection period, theouter two gated drops 58 and 63 are closed and the central four gateddrops 59-62 are opened. The primary or first stage injection pressurethen delivers plasticized material to the central portion of the moldcavity 30.

The particular sequencing of the six gates 64 in the preferredembodiment described herein was determined empirically. Alternatively,conventional mold fill analyses may be used to determine the appropriatesequencing of the gated nozzles to achieve the desired melt frontadvancement and fill balancing. It will be readily apparent that thegate sequencing that may be used in a particular application will dependon a variety of factors, including mold cavity shape, number of drops,and type of material used, to name only a few.

Once the part has been substantially filled (e.g. 95% filled) during thefirst stage pressure of the preferred embodiment, the outer two valvegates (64 of drops 58 and 63) open once again such that plasticizedmaterial is delivered to the mold cavity 30 through all six valve gates.The injection pressure is also lowered to a secondary or hold pressureof approximately 50% the primary injection pressure, whereby the moldmay continue to fill to capacity and to compensate for shrinkage duringcooling, without creating unwanted flash. The hold pressure, however, isstill sufficient to avoid appreciable expansion of the melt 22. Once themold has been completely filled, all six valve gates 64 are closed toseal the manifold from the mold cavity 30.

Because the manifold seal created by the valve pins 74 is much strongerthan a seal created by a thermal gate 42 as described above, thepositive mechanical gate shut off capability provided by the valve pinarrangement eliminates the need to decompress the melt 22 before,during, or after plastication. Indeed, the positive shut off provided bythe valve pin arrangement avoids drool at the nozzle locations 64without decompression of the melt 22. Therefore, a sufficientcompression pressure may be maintained on the melt pool 22 whenever thevalve gates 64 are closed, such as during and between part cycles forexample, to avoid appreciable expansion of the melt 22. As mentionedabove, expanding gasses or other volatiles in the melt 22 upstream ofthe valve gates 64 during melt decompression has been found to oftenresult in imperfections in subsequently-molded parts. The compressionpressure is therefore preferably of a sufficient magnitude to keep suchexpansion from occurring and thereby forming imperfections, such assplay for example, on the molded part. A compression pressure of atleast approximately 75-150 PSI (or 0.5-1.0 Mpa), for example, ispreferably used in the system described herein.

Moreover, the positive mechanical gate shut off feature allows the clampto be opened for part removal while additional material is plasticizedas a part of screw recovery, thereby further reducing overall cycletime. The compression pressure is preferably maintained on the melt 22during such screw recovery.

The total time for the sequential valve gate molding cycle describedabove is approximately 75 seconds or less, as compared to the 100-110second or more cycle previously experienced with non-sequential thermalgates. Not only does the reduced cycle time result in a savings in timeand energy, as well as an increase in manufacturing capacity, thereduction in cycle time also further enhances the quality of the finalmolded product. Indeed, decreased residence time of the melt 22 helps toavoid the occurrence of gas bubbles or other volatiles that may causesplay or other imperfections in the final product.

In addition to the reduction in overall cycle time and the occurrence ofsplay imperfections in the molded product, the preferred sequential fillvalve gated system provides control over the melt front advancementduring the filling phase. In turn, this provides more control over thefinal part size and shape by evenly distributing and reducing molded-instresses. With reference to the formation of an automobile bumper forexample, traditionally the center of the bumper mold fills first andbecomes overpacked as the wings of the bumper mold fill out. Thesequential fill valve gated system described herein permits the wings ofthe mold to be filled first, so as to avoid overpacking the mold center.This allows a fill pattern to be constructed whereby all the flow frontswithin the mold converge simultaneously. As a result, more uniformpacking may be achieved over the entire molded product to provide alower and more uniform stress distribution within the molded product.

Moreover, since the various flow fronts can be controlled to convergemore uniformly, knit line appearance can be reduced or eliminated toimprove the appearance of the molded product. Knit lines in earliermolded automobile facias, for example, often occurred in the center ofthe part, and were sometimes visible even after painting.

Empirical analysis of the sequential fill valve gated system describedherein determined that the imposition of a delay of approximatelythree-seconds before injecting into the mold cavity 30 through thecenter four gates (64 at drops 59-62) both moved and optimized thelocation of the knit line on the molded product, and reduced theintensity of the knit line such that any read-through after paintingcould be minimized and often eliminated.

Control of the flow front as described herein may also be used to reducethe occurrence of flash, which results in less trimming of the moldedpart and prevention of mold damage at the parting line 80. Moreover,instead of customizing the sizes of the various drop channels in themanifold to control the flow front and to balance the fill, thesystematic control of the valve gates 64 as provided by the systemdescribed herein may be used to provide the necessary flow front controland fill balancing using uniformly-sized interior channels in themanifold 50. Indeed, the interior channels in all six drops 58-63 usedin the system described herein are, for example, each one inch indiameter to correspond with a one-inch channel diameter upstream of thedrops 58-63 in the manifold 50. There is no longer a need to designand/or otherwise rely upon customized drops, which are often both costlyto and time-consuming to ready. Flow front control and fill balancing isinstead achieved through appropriate sequencing of the various valvegates 64, as provided by the present invention.

The sequential fill valve gated system described herein also serves toreduce the molded-in and localized stresses. Reduced stresses of thissort result in improved dimensional stability of the molded part.Indeed, the balanced fill can reduce molded-in and localized surfacestresses by equally distributing the pressure needed to fill certainregions of the mold, such as the wing regions 84 and 86 of the examplebumper component mold 30 shown in the figures. This avoids any need tomold a crown onto the molded part to otherwise compensate for stress andshrinkage effects, or part movement during paint curing at elevatedtemperatures.

The system of the present invention also serves to improve the paintadhesion characteristics of the molded product, which can be critical incertain molding applications such as automobile facias for example. Itis often required that the painted surface be capable of resistingchipping and peeling throughout the life of the molded part. Moldedproducts formed through the use of the system described herein haveevidenced improved paint adhesion characteristics, thereby reducing thetime and expense necessary to ensure that paint otherwise adheres to thepart.

The improved paint adhesion characteristics is attributed to the lowersurface stresses on the molded product, and to the more controlled andefficient mold cavity venting capable of being realized with the presentinvention. Under certain typical processing conditions, the surfacestructure of the base resin can be altered in such a way that paintadhesion is negatively effected. Specifically, the molded surface, underthe influence of high pressures, high temperatures, and entrappedvolatiles resulting from an unbalanced fill, becomes more chemicallyresistant to solvents required for paint preparation. The increasedcontrol over the fill pattern as provided by the sequential fill valvegated system described herein reduces molded-in stresses which, in turn,results in improved paint adhesion characteristics of the moldedproduct.

With the ability to control fill patterns and knit line locations, it isalso possible to successfully fill more complex mold cavity geometries.This increased molding window gives the design engineer more flexibilitywith molded parts such as automobile facias for example. Further, asindustries such as the automotive industry move toward molded-in colorfor large exterior and interior applications, the control provided bythe sequential fill valve gated system described herein over fillpatterns and knit line locations—which are important for molded-in colorapplications—offers the added processability to meet this challenge. Theelimination or reduction of knit lines in molded-in color parts, forexample, can be achieved by sequencing the valve pins 74 such that theoutboard nozzle opens first and, the next inboard gate opens after theflow front from the outboard nozzle passes the inboard gate location.Material from the inboard nozzle pushes through the flow front andadvances to the next adjacent inboard nozzle location. This processcontinues until the flow front passes the last nozzle location at whichthe last gate opens to finish filling the mold cavity 30. The result canbe the elimination or at least a reduction of knit lines, which canprove to be significant in the success of a molded-in color application.

By utilizing plastic pellets with the loading of nanoparticles discussedabove (e.g., less than 15% of a total volume of the pellets), highermodulus of elasticity of conventional large plastic parts can beachieved, and thus be manufactured with a reduced wall thickness whilemaintaining the same required impact resistance. Control over the meltfront advancement during the filling phase also makes it possible tosignificantly increase the number of nozzles or drops used to fill themold cavity 30. Additional nozzles may be used in this fashion to reducethe flow length versus wall thickness ratios otherwise required to fillthe mold cavity 30, which can in turn lead to thinner wall molding.Control over the filling pattern of the automobile fascia in combinationwith use of nanoparticle reinforced pellets described herein, forexample, may result in a reduction of a typical 3.3 mm fascia wallsection by more than 33%. In addition to making thinner walled parts, itis also possible to make larger parts by enlarging the size of moldcavity 30. Larger parts can thus be made while maintaining or reducingthe wall thickness of the parts. Control over the melt front alsoprovides for more efficient venting of the mold cavity 30, insofar asair trapped in the cavity 30 can be directed toward and out of theappropriate mold vents in a systematic manner.

In one example, the modulus of the material used to form a fascia isincreased to between about 200,000 to about 500,000 PSI (or 1378 to 3446Mpa). As a result, the fascia can be provided with a (largestdimension/wall thickness) ratio of greater than 1200. In one example, avehicle fascia having an average wall thickness of less than or equal to2.2 mm and largest dimension (from wheel well to wheel well) of at least3000 mm is provided while maintaining the required impact resistantcharacteristics. In this example, it can be appreciated that the moldcavity 30 has an average distance between the major facing surfaces 31and 33 of about 2.2 mm and a largest dimension of at least 3000 mm. Theaccuracy of the average wall thickness measurement is generally within+/−0.2 mm.

In another preferred example, a vehicle hood panel is provided with a(largest dimension/wall thickness) ratio of greater than 750. In oneexample, the hood panel has a largest dimension of at least 1800 mm andan average wall thickness of less than or equal to 2.5 mm.

In yet another example, a vehicle interior door panel is provided with a(largest dimension/wall thickness) ratio of greater than 500. In oneexample, the door panel has a largest dimension of at least 750 mm andan average wall thickness of less than or equal to 1.5 mm.

For these last two examples, the size of the mold cavity would bechanged accordingly.

The ratio's discussed above are dependent upon the structuralintegrity/impact resistance/elasticity requirements for the parts inquestion.

In accordance with the present invention, by adding the exfoliatedplatelet material in accordance with the above, the modulus of thelarge, thin part can be increased without significantly losing impactresistance. Because the modulus is increased, large thin parts, such asfascia, can be made thinner than what was otherwise possible. Morespecifically, fascia materials for automobiles must have sufficientimpact resistance or toughness to withstand various standard automotiveimpact tests. For example, an automotive fascia must withstand a typicaldart (puncture type) impact test wherein the fascia will not crack orpermanently deform upon impact of at least 200 inch pounds force (or22.6 Joules) at a temperature of −30° C. or lower. In a conventionalIZOD impact test, it is desirable for the fascia to withstand at least10 ft pounds/inch (or 535 Joules/meter) at room temperature and at least5 ft pounds/inch (or 267 Joules/meter) at −30° C. In order to withstandcracking at such force levels, the modulus for the conventional fasciais typically between about 70,000 to about 150,000 pounds per squareinch (PSI) (or 482 to 1034 Mpa).

In accordance with the present invention, the modulus can be increasedby a factor of 2 to 3 times, without significantly effecting the impactresistance.

In addition to the above mentioned benefits, use of the nanoparticlereinforced pellets enables the coefficient of linear thermal expansionto be reduced to less than 40×10−6 inches of expansion per inch ofmaterial per degree Fahrenheit (IN/IN)/°F. (or 72×10⁻⁶ (mm/mm)/°C.),which is less than 60% of what was previously achievable for motorvehicle fascia that meet the required impact tests. As a furtherbenefit, the surface toughness of the fascia can be improved.

The improved surface toughness provided by the nanoparticles greatlyreduces handling damage and part scrap. It also eliminates the need forthe extra packaging and protective materials and the labor involved.

In addition, it is possible to double the modulus of polymers withoutsignificantly reducing toughness. Thus, it is possible to produce partslike fascia using 20-35% thinner wall sections that will have comparableperformance. The use of nanoparticles can provide the mechanical,thermal, and dimensional property enhancements, which are typicallyobtained by adding 20-50% by weight of glass fibers or mineral fillersor combinations thereof to polymers. However, only a few percent ofnanoparticles are required to obtain these property enhancements.

As a result of the fact that such low levels of nanoparticles arerequired to obtain the requisite mechanical properties, many of thetypical negative effects of the high loadings of conventionalreinforcements and fillers are avoided or significantly reduced. Theseadvantages include: lower specific gravity for a given level ofperformance, better surface appearance, toughness close to that of theunreinforced base polymer, and reduced anisotropy in the molded parts.

It is preferable for these parts to have reinforcement particles of thetype described herein comprising about 2-10% of the total volume of thepanel, with the balance comprising the thermoplastic (preferablypolyolefin) substrate. It is even more preferable for these exteriorpanels to have reinforcement particles of the type contemplated hereincomprising about 3%-5% of the total volume of the panel.

In accordance with another specific embodiment of the present invention,it is contemplated that the injection molding apparatus can be used tomake large, highly reinforced parts having a modulus of elasticity of1,000,000 PSI (or 6892 Mpa) or greater. Conventionally, these partstypically require loadings of 25-40% by volume of glass fiberreinforcement. This amount of glass fiber loading would result in a highviscosity of any melt pool that could be used in the injection moldingapparatus of the present invention and would thus render the injectionmolding apparatus disclosed herein largely impractical for suchapplication.

Use of the plastic pellets 10 enables the injection molding apparatusdisclosed herein to manufacture large parts that can be provided withimpact resistance characteristics that were not previously attainable.For example, the injection molding system of the present invention isable to manufacture large parts having a modulus of elasticity ofgreater than 1,000,000 PSI (or 6892 Mpa) by use of the plastic pelletsreinforced with loadings of 8-15% by volume of nanoparticles, with atleast 70% of the nanoparticles have a thickness of 10 nanometers orless. As with the above described embodiment, the pellets used hassubstantially the same material composition as the part to bemanufactured. Specifically, the pellets have a modulus of elasticity ofgreater than 1,000,000 PSI (or 6892 Mpa) and have loadings of 8-15% byvolume of nanoparticles, with at least 70% of the nanoparticles having athickness of 10 nanometers or less.

In this case of molding large parts with a modulus of elasticity greaterthan 1,000,000 PSI (or 6892 Mpa), it may be desirable to use engineeringresins instead of polyolefins. Such engineering resins may includepolycarbonate (PC), acrylonitrile butadiene styrene (ABS), a PC/ABSblend, polyethylene terephthalates (PET), polybutylene terephthalates(PBT), polyphenylene oxide (PPO), or the like. Generally, thesematerials in an unreinforced state has a modulus of elasticity of about300,000 PSI-350,000 PSI (or 2068-2412 Mpa). At these higher loadings ofnanoparticles (8-15% by volume), impact resistance will be decreased,but to a much lower extent than the addition of the conventional 25-40%by volume of glass fibers.

Although certain embodiments of the invention have been described andillustrated herein, it will be readily apparent to those of ordinaryskill in the art that a number of modifications and substitutions can bemade to the sequential fill valve gated injection molding systemdisclosed and described herein without departing from the true spiritand scope of the invention.

What is claimed is:
 1. A method for molding large components, comprisingthe steps of: providing reinforced plastic pellets comprising at leastone thermoplastic material and reinforcement particles dispersed withinthe at least one thermoplastic material, the reinforcement particlescomprising less than 15% of a total volume of the pellets and at least40% of the reinforcement particles having a thickness of less than about50 nanometers, said pellets having a modulus of elasticity greater than6892 MPa, said reinforcement particles comprising 8-15% by volume ofsaid total volume of said pellets, and at least 70% of saidreinforcement particles having a thickness of 10 nanometers or less;melting the reinforced plastic pellets to produce plasticized materialtherefrom; communicating said plasticized material through a manifold toa cavity mold, said manifold having at least two spaced valve gates thatare independently opened and closed as directed by a controller toselectively communicate said plasticized material through said manifoldto said mold cavity at separate locations in the mold; applying aprimary injection pressure to said plasticized material in said manifoldto fill said mold cavity through sequential opening and closing of saidvalve gates as directed by said controller; applying a secondaryinjection pressure to said plasticized material in said manifold tocontinue to fill said mold cavity, said secondary injection pressurebeing less than said primary injection pressure; closing said valvegates to seal said manifold from said mold cavity when said mold cavityis filled; and holding said plasticized material within said manifold incompression while said mold cavity is open for removal of said moldedcomponent from said mold cavity, said compression being maintained withthe assistance of said closed valve gates to prevent appreciableexpansion of said material.
 2. A method for molding large components asset forth in claim 1, further comprising the step of: plasticizingadditional material while said mold cavity is open for removal of saidmolded component from said mold cavity, said additional plasticizedmaterial being held for anticipated communication through said manifoldinto said mold cavity during a subsequent molding cycle, and saidadditional plasticized material being held in compression with theassistance of said closed valve gates to prevent appreciable expansionof said material.
 3. A method for molding large components as set forthin claim 1, wherein said controller directs all of said valve gates toopen for simultaneous transfer of plasticized material through saidvalve gates into said mold cavity while said secondary injectionpressure is applied to said plasticized material in said manifold.
 4. Amethod for molding large components as set forth in claim 3, whereinsaid secondary injection pressure is applied with the aid of a screwfrom an injection molding machine, and wherein occurrence of saiddirection from said controller to all of said valve gates to open forsimultaneous transfer of plasticized material through said valve gatesis a function of both molding cycle time and position of said screw. 5.A method according to claim 1, wherein said reinforcement particles areformed by exfoliating larger mineral particles into constituent layersso that said at least 40% of the reinforcement particles have saidthickness of less than about 50 nanometers.
 6. A method according toclaim 1, wherein at least 50% of the reinforcement particles have athickness of less than 10 nanometers.
 7. A method according to claim 6,wherein at least 70% of the particles have a thickness of less than 5nanometers.
 8. A method according to claim 1, wherein said thermoplasticcomprises at least one polyolefin material.
 9. A method according toclaim 1, wherein said thermoplastic comprises at least one enginerringresin material.
 10. A method according to claim 1, wherein at least 50%of the reinforcement particles have a thickness of less than about 20nanometers, at least 90% of the reinforcement particles have a thicknessof less than about 10 nanometers, and at least 99% of the reinforcementparticles have a thickness of less than about 30 nanometers.
 11. Amethod according to claim 1, wherein said mold cavity is defined betweentwo major facing surfaces, and wherein an average distance between saidmajor facing surfaces is about 2.2 mm+/−0.2 mm, and wherein said moldcavity has a largest dimension of at least 3000 mm, such that saidmolded component has an average wall thickness of about 2.2 mm+/−0.2 mmand has a largest dimension of at least 3000 mm.
 12. A method accordingto claim 1, wherein said mold cavity is defined between two major facingsurfaces, and wherein an average distance between said major facingsurfaces is about 1.5 mm+/−0.2 mm, and wherein said mold cavity has alargest dimension of at least 750 mm, such that said molded componenthas an average wall thickness of about 1.5 mm+/−0.2 mm and has a largestdimension of at least 750 mm.
 13. A method according to claim 1, whereinsaid mold cavity is defined between two major facing surfaces, andwherein an average distance between said major facing surfaces is about2.5 mm+/−0.2 mm, and wherein said mold cavity has a largest dimension ofat least 1800 mm, such that said molded component has an average wallthickness of about 2.5 mm+/−0.2 mm and has a largest dimension of atleast 1800 mm.
 14. A method for molding large components, comprising thesteps of: providing reinforced plastic pellets comprising at least onethermoplastic material and reinforcement particles dispersed within theat least one thermoplastic material, the reinforcement particlescomprising less than 15% of a total volume of the pellets, and at least40% of the reinforcement particles having a thickness of less than about50 nanometers, said pellets have a modulus of elasticity greater than6892 MPa, said reinforcement particles comprising 8-15% by volume of atotal volume of said pellets, and at least 70% of said reinforcementparticles having a thickness of 10 nanometers or less; melting thereinforced plastic pellets to produce plasticized material therefrom;using a manifold for communicating said plasticized material to a cavitymold, said manifold having at least two spaced valve gates that areindependently opened and closed as directed by a controller toselectively communicate said plasticized material from said manifold tosaid mold cavity at separate locations in the mold; applying a primaryinjection pressure to said plasticized material in said manifold to fillsaid mold cavity through sequential opening and closing of said valvegates as directed by said controller; applying a secondary injectionpressure to said plasticized material in said manifold to continue tofill said mold cavity, said secondary injection pressure being less thansaid primary injection pressure; closing said valve gates to seal saidmanifold from said mold cavity when said mold cavity is filled; andholding said plasticized material within said manifold in compressionwhile said mold cavity is open for removal of said molded component fromsaid mold cavity, said compression being maintained with the assistanceof said closed valve gates to prevent appreciable expansion of saidmaterial.
 15. A method for molding large components as set forth inclaim 14, further comprising the step of: plasticizing additionalmaterial while said mold cavity is open for removal of said moldedcomponent from said mold cavity, said additional plasticized materialbeing held for anticipated communication through said manifold into saidmold cavity during a subsequent molding cycle, and said additionalplasticized material being held in compression with the assistance ofsaid closed valve gates to prevent appreciable expansion of saidmaterial.
 16. A method for molding large components as set forth inclaim 14, wherein said controller directs all of said valve gates toopen for simultaneous transfer of plasticized material through saidvalve gates into said mold cavity while said secondary injectionpressure is applied to said plasticized material in said manifold.
 17. Amethod for molding large components as set forth in claim 16, whereinsaid secondary injection pressure is applied with the aid of a screwfrom an injection molding machine, and wherein occurrence of saiddirection from said controller to all of said valve gates to open forsimultaneous transfer of plasticized material through said valve gatesis a function of both molding cycle time and position of said screw. 18.A method according to claim 14, wherein said reinforcement particles areformed by exfoliating larger mineral particles into constituent layersso that said at least 40% of the reinforcement particles have saidthickness of less than about 50 nanometers.
 19. A method according toclaim 14, wherein at least 50% of the reinforcement particles have athickness of less than 10 nanometers.
 20. A method according to claim19, wherein at least 70% of the particles have a thickness of less than5 nanometers.
 21. A method according to claim 14, wherein saidthermoplastic comprises at least one polyolefin material.
 22. A methodaccording to claim 14, wherein said thermoplastic comprises at least oneengineering resin material.
 23. A method according to claim 14, whereinat least 50% of the reinforcement particles have a thickness of lessthan about 20 nanometers, at least 90% of the reinforcement particleshave a thickness of less than about 10 nanometers, and at least 99% ofthe reinforcement particles have a thickness of less than about 30nanometers.
 24. A method according to claim 14, wherein said mold cavityis defined between two major facing surfaces, and wherein an averagedistance between said major facing surfaces is about 2.2 mm+/−0.2 mm,and wherein said mold cavity has a largest dimension of at least 3000mm, such that said molded component has an average wall thickness ofabout 2.2 mm+/−0.2 mm and has a largest dimension of at least 3000 mm.25. A method according to claim 14, wherein said mold cavity is definedbetween two major facing surfaces, and wherein an average distancebetween said major facing surfaces is about 1.5 mm+/−0.2 mm, and whereinsaid mold cavity has a largest dimension of at least 750 mm, such thatsaid molded component has an average wall thickness of about 1.5mm+/−0.2 mm and has a largest dimension of at least 750 mm.
 26. A methodaccording to claim 14, wherein said mold cavity is defined between twomajor facing surfaces, and wherein an average distance between saidmajor facing surfaces is about 2.5 mm+/−0.2 mm, and wherein said moldcavity has a largest dimension of at least 1800 mm, such that saidmolded component has an average wall thickness of about 2.5 mm+/−0.2 mmand has a largest dimension of at least 1800 mm.